I believe this issue is specifically for Debian servers. I found an old article that explains why this behavior occurs on Debian machines: http://oscarm.org/2006/09/debian-php5-and-session-garbage-collection/
The above article still seems to be relevant, even though it seems to be marked as solved in the Debian bug system (http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=388808)
I moved my sites from a Centos machine to a Debian in April and under that time the number of session files has been growing.
I post this as a heads up for others running ProcessWire on Debian.
//Jasper
formmailer's post in Too many files in assets/sessions was marked as the answer